Once daily milking in late lactation : effects of somatic cell counts, milk yield and composition of dairy cows with high or low somatic cell counts : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Animal Science at Massey University

The present study was carried out in March 1993. Thirty six cows were used, 18 with initial low somatic cell count (ILSCC 250,000 cells/ml). Nine cows from each group were randomly allocated to once or twice daily milking for the four weeks of treatment period. All cows were then milked twice daily for the two weeks of post treatment period. The yield of milk, fat, protein and lactose were significantly reduced by once daily milking (P < 0.01) both in the ILSCC and the IHSCC groups. However, the effects of once daily milking on yields was larger for infected cows (Interaction P < 0.01). Once daily milking significantly increased the concentration of protein (P < 0.001) in both groups and fat (P < 0.01) in only the ILSCC group. On the other hand, once daily milking significantly reduced the concentration of lactose, the effect being larger in the IHSCC cows (Interaction P < 0.001). SCC was increased by once daily milking, but the effect was significant only in the IHSCC group (Interaction, P < 0.10). The total number of somatic cells produced per day was not affected by once daily milking. Similarly, post-treatment analysis results showed no carry over effect of once daily milking on yield and SCC of milk from either ILSCC or IHSCC cows. lvlilk composition results showed a carry over effect of once daily milking on protein (P < 0.05) and lactose (P < 0.1) for ILSCC and IHSCC cows respectively. There was no carry over effect recorded for the concentration of fat. lV These results show that SCC can be used with confidence to identify subclinically infected cows in later lactation milked once or twice a day. The increase in SCC in cows with infection is probably due to the reduced volume of milk, since the total number of somatic cells secreted into the milk per day was not affected by once daily milking treatment (Interaction P < 0.5). The results also show that herds of cows which maintain a low incidence of subclinical mastitis can be milked once daily in late lactation, with much less effect on yield of milk and its constituents than would be expected in those with higher incidence of infection